1. Field of the Invention
The present invention relates to a method for transmitting signals in a projection system.
More particularly, the present invention relates to a method for transmitting signals in a projection system, including the step of transmitting signals to two or more projection elements, which projection elements each are used for projecting a plurality of colors, at least one of these projection elements having a different polarization state for at least one of the colors projected by the respective projection element.
2. Discussion of the Related Art
Passive stereo projection is a known technique which can be used to create a stereo or a so-called 3D or stereoscopic image effect and which can, for instance, be implemented with at least two LCD or LCoS projectors.
In the known projection systems which are used to create such a passive stereo effect based on polarization, at least two projectors are set up in such a way that one of the projectors receives only the video signals with the images for the left eye, and a second projector receives the video signal with the images for the right eye, whereby said video signals contain the image information and consist of information for the red, green and blue color, which forms a colored image together.
When, for example, two projectors are used, both these projectors will project light with a different polarization state on preferably a non-depolarizing screen.
However, the polarization state of the three colors red, green and blue, which are projected by each individual projector, is the same for each color.
With a pair of glasses, equipped with substantially orthogonally polarized filters, respectively, one filter for the left eye and one filter for the right, the left eye will receive only the images from one projector, while the right eye will receive only the images from the other projector.
It is known that LCD or LCoS projectors, which are used for passive stereo projection, are equiped with extra optical elements in order to obtain that the light from one projector has an equal polarization state for the three colors red, green and blue.
The images for a single set of passive stereo projectors can be supplied via, on the one hand, a set of two passive stereo signals or, on the other hand, via a single active stereo signal.
In the first case of a set of two passive stereo signals, an image generator transmits, both the left image and the right image, via an individual electrical path to each one of the two projectors.
In the other case, a single active stereo signal is used, in which the left and right images follow each other in time, whereby one image contains a left and a right field and whereby the frequency of such a signal is twice as high as for a passive stereo signal, which is a single field image signal.
When such a single active stereo signal is used, in combination with a set of two passive stereo projectors, said active stereo signal has to be distributed to both of the projectors, whereby only the correct fields have to be extracted from the signal, for instance the left fields for the first projector and the right fields for the second projector.
This means that, in applications with N sets of two passive stereo projectors working together to provide a big compound image, 2N passive stereo signals or N active stereo signals have to be provided by one or more image generators.
A method is already known, which makes use of the above-mentioned technique, whereby the first projector has linearly polarized light with the same polarization direction for the three colors, and the second projector has also linearly polarized light for the three colors, but with a polarization direction which is, for instance, altered in such a way, that it will be substantially orthogonal to the polarization direction of the first projector. In this case, the change of the polarization direction can be realized by using a broadband halfwave retarder.
In a second known method, the polarization state of the three colors in the first projector is right-handed circular polarization, and the polarization state of the three colors in the second projector is left-handed circular polarization.
However, in most cases the light of the three colors of LCD and LCoS projectors does not have the same polarization state for each color, because this facilitates an efficient optical design of said LCD or LCoS projector.
For instance, often an SPS-type recombination cube is used, which causes the light of, for example, the green color to be differently polarized than the light of the red and blue color, moreover in this example of the SPS cube, the green light is orthogonally polarized with respect to the light from the red and blue color.
In this case special optical elements have to be integrated in the LCD projector or have to be added to the LCD projector to obtain a corresponding polarization state of the three colors.
A first optical element which may be integrated to this aim, is a so-called SSS-type recombination cube, where the polarization state of the light is equal in the three color channels.
A disadvantage of the implementation of such a SSS-type recombination cube, is that it is known to have a low output efficiency, for example the light output might drop with 20%.
A second optical element which may be implemented, is an extra polarizer, which is placed at an angle of 45 degrees, both with respect to, for the above-mentioned example, the S-polarization state of red and blue, and the P-polarization state of green.
It is known that the usage of such an extra polarizer leads to a reduction of the light output with at least 50%.
A third optical element which is used to obtain a corresponding polarization state of the three colors, is a color selective retarder, which can change the polarization state of one color with respect to the other colors.
A disadvantage of such color selective retarders, is that they consist of a large stack of optimized retarders, which also introduce an important loss of light.